1. Field of the Invention
This invention relates to new and improved articulated joint for flowlines and more particularly relates to a universal joint assembly of the type for a multiple conduit system used in underwater oil or gas production.
2. Description of the Prior Art
Production of crude oil from deep water areas remote from land poses significant problems. For example, in very deep water it is not economically feasible to install rigid ocean floor supported platforms of the type used to support production operations in shallow water. Also, the laying of large diameter pipelines in deep water is often impractical and unattractive economically. One promising approach for overcoming these difficulties involves the use of a production system comprising a production manifold positioned on or near the ocean floor and a riser assembly extending from the production manifold to an overlying moored floating vessel. The floating vessel is provided with conventional processing equipment and a storage area for crude oil. The riser assembly includes multiple flexible flowlines interconnecting the vessel with the subsea wellheads and manifold systems.
Development of a riser assembly that will withstand the forces encountered in typical floating production operations poses severe design problems. The riser system must have sufficient flexibility and structural strength to accommodate motions and withstand forces resulting from vessel excursions and from wave and current action. The riser must also have sufficient flow lines to conduct fluids from the sea bottom to the vessel, to conduct separated water or gas back to the sea bottom and to conduct flowline tools between the vessel and the wellheads.
A number of riser systems have been suggested for subsea petroleum production. One such riser system, commonly designated a "rigid self-standing riser", has a rigid connection at the wellhead or manifold and a flowline housing tensioned by a suitable buoyancy chamber at a distance of about 200 feet (61 meters) below the water surface. The steel flowlines in the flowline housing are then connected from the top of the buoyancy chamber to a moored floating vessel through a bundle of flexible floating hoses. This arrangement requires no special multiline flexible joint to provide flexibility riser but has the disadvantage that it requires heavy ballast and pile anchors to overcome the high tension and bending loads from riser buoyancy and current drag. Another disadvantage is that the flexible hoses are exposed to severe environmental conditions and are vulnerable to damage by surface vessels.
Another more suitable type of production riser, commonly designated a "tension leg riser", uses a single buoy, one multiline universal joint directly below the buoy, a riser stem housing, a second multiline universal joint directly above the base at the ocean floor and a multiple number of flow lines extending from the buoy to the ocean floor. The riser moors the production and storage vessel through a multiline swivel and articulated arm arrangement that permits the vessel to rotate or "weathervane" around the buoy in response to wave, wind and current action. This type of riser can be constructed using rigid flow lines to avoid the problems associated with the use of flexible flowlines. The multiline universal joints form a vital part of the production riser in that they provide the riser system with the requisite flexibility and structural strength to enable it to withstand the severe environmental conditions encountered in deep water operations.
In designing articulated joints, such as universal joints for riser systems having rigid flowlines, a number of factors must be taken into consideration, including for example, environmental conditions, water depth and the number and sizes of the flow lines. A significant economic and operational consideration is the size of the universal joint. In prior multiline universal joints, only two of the rigid flow lines could pass through the universal joint on the axes of pivot. The remaining rigid flow lines were positioned along the side of the universal joint. Each of the flow lines positioned along the side of the universal joint was provided with fluid swivels at the pivot axes of the universal joint. Because the fluid swivels were required to be located in end to end relation along the axes of pivot, the universal joints for a riser having a large number of flowlines had, by necessity, the fluid swivels extending a substantial distance outside the universal joint. Each additional flow line required an additional fluid swivel on each axis of pivot thereby increasing the overall size of the universal joint. Unfortunately, the larger sized universal joints are more expensive, heavier, more difficult to install, and are more susceptible to greater environmental loading. Further, it is difficult to maintain all of the fluid swivels in alignment with the pivot axes of the universal joint. Misalignment causes high stresses in the swivel bearings and results in early fatigue failure of the swivel. Further, in order to replace a fluid swivel immediately adjacent to the universal joint, it was necessary to first remove each of the outer fluid swivels on the axes of pivot. Since the number and sizes of the flow lines are controlled by operating conditions, it is apparent that a need exists for a multiline universal joint that will minimize the stresses imposed on the fluid swivels and provide a relatively compact flexible connection between multiple rigid flow lines to minimize environmental loading and reduce the cost.